The 3rd Generation Partnership Project (3GPP) is specifying the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) consisting of the Long Term Evolution (LTE) and System Architecture Evolution (SAE) concepts. The architecture of the LTE system is shown in FIG. 1.
LTE dual connectivity is a feature defined from the user equipment (UE) perspective, where the UE may simultaneously receive data from and transmit data to two different eNBs. This feature is part of 3GPP Rel-12. The two different eNBs are usually denoted as Master eNodeB (MeNB) and Secondary eNodeB (SeNB). Operating on different frequencies, the MeNB and SeNB provide separate cell groups for the UE, a master cell group (MCG) and a secondary cell group (SCG).
The protocol architecture for Release 12 (Rel-12) of the 3GPP specifications for LTE, as shown in FIG. 2, supports three types of radio bearers: MeNB bearers, split bearers, and SeNB bearers. In pre-Rel-12 LTE, i.e., before the introduction of dual connectivity, handover failures (HOF) and radio link failures (RLF) are detectable by the UE. The UE tries to reestablish its connection upon detection of such a failure. Otherwise, it goes to IDLE.
In detail, the following detection methods were considered. A handover failure timer, such as a T304 timer, is started upon reception of a radio resource control (RRC) connection reconfiguration message that includes mobility control information. The T304 timer is stopped upon successful random access to a target handover cell. Expiration of the T304 timer triggers a procedure for reestablishment of the RRC connection. A reestablishment timeout timer, such as a T311 timer, is started when the UE tries to reestablish RRC connection, and the T311 timer is stopped when successfully reestablished. At expiration of the T311 timer, the UE goes to IDLE or declares that reestablishment is unsuccessful. A detection of physical-layer (PHY) failure timer, such as a T310 timer, is started when a physical layer problem is indicated by physical layer, and the T310 timer is stopped when the physical-layer failure is indicated as being resolved by the physical layer. Upon expiration of the physical-layer failure timer, reestablishment of an RRC connection is triggered. The T310 timer can only be started if reestablishment is not already ongoing (i.e., only if T311 is not running), and only after successful handover (i.e., only if T304 is not running).
Other failure detection methods include a random access procedure (random access channel or RACH) failure that triggers reestablishment of an RRC connection if reestablishment is not already ongoing (T311 is not running), and only after successful handover (T304 is not running). A radio link control (RLC) failure triggers reestablishment of an RRC connection, if reestablishment not already ongoing (T311 is not running) RLC failure cannot happen when T304 is running, since a random access procedure is pending during that time period and RLC retransmissions have not yet been started.
For dual connectivity, a new event, known as secondary-cell-group failure (SCG failure) or SeNB radio-link failure (S-RLF), has been introduced. When this event is triggered, the UE stops uplink transmission in the SeNB, suspends SCG and split bearers, and sends an indication to the MeNB. The indication is sent via an RRC message in uplink and can be called a “UE failure indication,” which can be regarded as an “SCG Failure Information message.
The following are considered to be SCG failure-triggering events. An SCG change failure timer (also known as a T307 timer), is started when SCG change is triggered, and stopped after successful random access to a new SCG. The SCG change failure timer is similar to the pre-Rel-12 T304 handover failure timer, but being related to the SCG, the procedure similar to handover is called SCG change. If the SCG change failure timer expires, the UE triggers failure indication, with an RRC message to the MeNB. An SCG physical layer (PHY) failure timer (also known as a T313 timer), is started when an SCG PHY problem is detected, but SCG change is currently not ongoing (the SCG change failure timer is not running) The SCG PHY failure timer is similar to the pre-Rel-12 radio link failure timer T310, but the SCG PHY failure timer is related to the SCG. This SCG PHY failure timer is stopped when an SCG physical layer problem is indicated as being resolved by SCG PHY. If the SCG PHY failure timer expires, the UE triggers failure indication in an RRC message to the MeNB. SCG MAC failure triggers the sending of a UE failure indication in an RRC message to the MeNB, if SCG change is not currently ongoing (i.e., if the SCG change failure timer is not running) SCG MAC failure occurs when the UE has repeated random access procedures and preamble transmissions more than a configurable threshold. SCG RLC failure triggers the sending of a UE failure indication RRC message to the MeNB, but SCG RLC failure cannot happen while the SCG change failure timer is running SCG RLC failure occurs when the number of RLC retransmissions exceeds a configurable threshold.
In each of the failure events, it is assumed that the failure type is indicated in the RRC message. It is currently unclear how many UE failure indication RRC messages are sent (and when) to the MeNB for the concurrent SCG failure detection methods defined above. Too many UE failure indications may waste radio resources and load the MeNB unnecessarily.